Cross River Gorillas

The Cross River gorilla is a subspecies of the western gorilla. It was named a new species in 1904 by Paul Matschie, a mammalian taxonomist working at the Humboldt University Zoological Museum in Berlin. With an estimated 280 individuals left in the world, the Critically Endangered Cross River gorilla is Africa’s most threatened great ape. Living in the highlands of Cameroon on the border with Nigeria, they have been hunted for years by local residents, making them very wary of humans..

It is the most western and northern form of gorilla, and is restricted to the forested hills and mountains of the Cameroon-Nigeria border region at the headwaters of the Cross River -Nigeria. It is separated by about 300 km from the nearest population of western lowland gorillas and by around 250 km from the gorilla population in the Ebo Forest of Cameroon. Estimates from 2014 suggest that fewer than 250 mature Cross River gorillas remain, making them the world’s rarest great ape. Groups of these gorillas concentrate their activities localities across a 12,000 km2 (4,600 sq mi) range, though recent field surveys confirmed the presence of gorillas outside of their known localities suggesting a wider distribution within this range. This distribution is supported by genetic research, which has found evidence that many Cross River gorilla localities continue to maintain contact through the occasional dispersal of individuals.

The Cross River gorilla, like many other gorilla species, prefer a dense forest habitat that is uninhabited by humans.

Due to the Cross River gorilla’s body size they require large and diverse areas of the forest to meet their habitat requirements. Similar to most endangered primates, their natural habitat exists where humans are often occupying and using for natural resources. Forests that are inhabited by the Cross River gorilla vary in altitude from approximately 100 to 2,037 metres (328 to 6,683 ft) above sea level.

The habitats of the cross river gorilla are negatively affected by the drastic deforestation and fragmentation of the land. These unfortunate events leave the gorilla species with few options for survival. As a result of deforestation and fragmentation, there are drastic reductions in carrying capacity, in other words, the size of the territories these animals inhabit has been significantly reduced. Because the population of humans living in this area is high, the amount of resources available to the cross river gorillas’ is limited. Even though this decrease in the availability of land may appear to be a problem, research studies have found that an adequate amount of rainforest still remains that is suitable and comfortable for this species. If, however, human pressures and activities towards deforestation continue, these territories will continue to diminish and ultimately will not exist. Additional examples of human activity that threaten cross river gorillas and, of course, other species, are hunting, logging, agriculture, fuel wood harvesting, clearance of lands for plantation and exploitation of natural resources. Gorillas and other primates are only a small part of the larger ecosystem and thus, they rely on many aspects of their habitat for survival. Furthermore, also because of their body size, they lack ability to adapt to new environment and they have a rather slow reproductive rate. Even though there is somewhat of a limited research on cross river gorillas, there is enough to conclude that these animals are currently able to sustain survival.

Most of the habit regions for cross river gorillas are legally protected due to their critically endangered status. However, there are still areas that are not like between the Kagwene Mountain and Upper Mbulu, and around Mone North.

Behavior

Cross River gorillas have certain nesting behaviors (i.e. mean nest group size, style of the nest, location of the nest, and nest reuse patterns) that depend on things such as their current habitat, climate, food source availability and risk of attack or vulnerability. According to research done on the Cross River gorillas living in the Kagwene Gorilla Sanctuary, there is a high correlation between whether a nest is constructed on the ground or in a tree and the season. From April up until November, Cross River gorillas are more likely to build their nests within a tree, and from November on they are more likely to build it on the ground. Overall, it was found that more nests built at night were built on the ground as opposed to in trees. This species is also more likely to construct nests during the wet season than the dry season, as well as construct more arboreal nests in the wet season. It was found that day nest construction was more common, especially in the wet season. Reuse of nesting sites was also found to be common, although it did not have any relation to the season. And, their mean nest group size is from four to seven individuals. Although, nest group size varies depending on the location of the species.

The groups of cross river gorillas consist mainly of one male and six to seven females plus their offspring. Gorillas in lowlands are seen to have less offspring than those in the highlands. This is thought to be because of the hunting rate in the lowlands and infant mortality rate. The groups in the highlands are densely populated compared to those in the lowlands.

The cross river gorilla’s diet consists largely of fruit, herbaceous vegetation, liana, and tree bark. Much like their nesting habits, what they eat is contingent on the season .Observations of the gorilla indicate that it seems to prefer fruit, but will settle for other sources of nutrition during the dry season of about 4–5 months in northern region.Cross River gorillas eat more liana and tree bark throughout the year, and less fruit during dry periods of scarcity.

Diet

The Cross River gorilla usually lives in small groups of 4-7 individuals with a few males and a few female members. Their diet usually consists of fruit, but in fruit scarce months, (August–September, November–January) their diet is primarily made up of terrestrial herbs, and the bark and leaves of climber and trees. Many of the Cross River gorilla food sources are very seasonal and thus their diets are filled with very dense, nutritious vegetation that is usually found near their nesting sites. It was found that the Afi Mountain group of Cross River gorilla diet mostly consisted. (Zingiberaceae) herbs, but when available in the wet season, they preferred to eat Amorphophallus difformis (Araceae) over the Aframomum, showing preference for certain foods that were seasonal and also an affinity to the vegetation that was only found in their habitat.

Nesting

The nesting behavior of the Cross River gorilla was influenced by the environmental conditions, such as the climate, predation, herbaceous vegetation, absence of suitable nest building materials and seasonal fruits nearby. The gorillas did portray certain nesting habits like mean nest group sizes, size and type of nest created, as well as the reusing of certain nesting location nearby seasonal food sources. In Sunderland-Groves research on the nesting behaviour of G. gorilla diehli at Kagwene Mountain they discovered that the nesting locations, whether on the ground or arboreal were greatly influenced by the current season, during the dry season most of the nests were made on the ground, yet during the wet season the majority of the nest were made high up in the trees, to provide protection from the rain. It was also found that the gorillas created more day nest during the wet season and reused nesting sites about 35% of the time. It was also found that the mean group size was 4-7 individual, yet the mean nest size at the sites was 12.4 nests and the most frequent number of nests was 13, showing some gorillas may have made multiple nests. The researchers also found nest sites with up to 26 nests, showing that sometimes multiple groups would nest together.

Aggression

The Cross River gorilla at the Kagwene Mountain in Cameroon has been observed using tools and it seems to be unique to the population in this region. They have been observed in three separate cases, in which they threw grass at the researchers, a detached branch and in a third case, in which an encounter with a man, who threw rocks at them led them to throw back fistfuls of grass. The entire encounter had the gorillas in the group observe the researchers and react to their presence with vocalizations then led to calm behaviour in the parts of the gorillas and finally an approach by the male gorillas and the throwing of grass at the researcher. The researchers have stated that this throwing behaviour might have arisen due to human contact in the fields and farms surrounding the mountain and the ambivalent nature of the gorillas is due to the surrounding people not hunting the gorillas due to the folklore about the gorillas.

Geographical distribution

This subspecies is populated at the border between Nigeria and Cameroon, in both tropical and subtropical moist broadleaf forests which are also home to the Nigeria-Cameroon chimpanzee, another subspecies of great ape. The Cross River gorilla is the most western and northern form of gorilla, and is restricted to the forested hills and mountains of the Cameroon-Nigeria border region at the headwaters of the Cross River. It is separated by about 300 km (190 mi) from the nearest population of western lowland gorilla (Gorilla gorilla gorilla), and by around 250 km (160 mi) from the gorilla population in the Ebo Forest of Cameroon.

The occurrence of Cross River gorillas has been confirmed in the Mbe Mountains and the Forest Reserves of Afi River, Boshi Extension, and Okwanggo of Nigeria’s Cross River State, and in the Takamanda and Mone River Forest Reserves, and the Mbulu Forest, of the Cameroon’s South West Province. These locations cover a mostly continuous forest area of about 8,000 km2 (3,100 sq mi) from Afi Mountain to Kagwene Mountain .

Habitat loss

Cross River gorillas reside in small populations split from other sub-populations of the species. They occupy roughly 14 apparently geographically separated areas in a landscape of approximately 12,000 km2 (4,600 sq mi) of rugged terrain spanning the Nigeria–Cameroon border region with population sizes estimated at 75–110 in Nigeria, and 125–185 in Cameroon.

Other sources of degradation such as hunting posed a much higher threat but habitat loss is now posing a much bigger threat to the species and their survival. Populations reside in areas of undisturbed dense forest which is scarce due to human occupation or use for natural resources. The Takamanda National Park and the Kagwene Gorilla Sanctuary are where most of the surviving members reside.

Nest distribution is influenced by anthropogenic factors within the sanctuary, with the disturbed southern section of the park avoided. Even though current wildlife laws regarding the areas are in place Cross River gorillas will not nest in areas near humans. Conservation and Eco-guards are empowered, by the government, to enforce wildlife laws within the sanctuary.

Fragmented population

The increased population of human inhabitants and the expansion of grasslands due to human activity has caused a fragmentation of the species into many sub-populations. Many factors mostly related to human activity contributed to the fragmentation of the population, including the expansion of farmland, human occupation, lack of accessible habitat and the sparsity of suitable or favorable habitat. Due to this isolation, gene flow has begun to slow and sub-populations are suffering from a lack of gene diversity which could mean a long term issue.

Hunting

A more recent phenomenon of the commercialization of bushmeat hunting has caused a large impact on the population. The hunting seems to be more intense within the lowlands and may have contributed to the concentration of gorillas within the highlands and their small population sizes. Despite laws preventing hunting it still persists due to local consumption and trade to other countries. The laws are rarely effectively enforced and due to the nature of the Cross River gorillas state any hunting has a large impact on the population and their survival.

Did you know that

Cross river Gorillas are very closely related to all gorilla species, and to chimpanzees, and they share over 98% of their DNA with humans.

Cross River gorillas are a subspecies of the western gorilla whereas mountain gorillas are a subspecies of the eastern gorilla.

Physical differences between Cross River gorillas and mountain gorillas are that Cross River gorillas have a more slender build and hair that is lighter in colour.

Threats to Cross- River Gorillas
Numerous dangers threaten the existence of the Cross River gorillas:

  1. Conversion of habitat for agriculture and grazing
  2. Road construction by logging companies
  3. Encroachment of surrounding farmland, which threatens to divide in two the Cross River National Park in Nigeria
  4. Poorly enforced wildlife protection laws, which leaves hunting unregulated
  5. Risk of inbreeding due to population isolation
  6. Transmission of fatal human diseases, specifically the Ebola virus